In actual line-change verification, we found that the release rate of SAC305 Solder Paste remains stable at 98.2% when the stencil aperture is larger than 0.3 mm. However, when the pad pitch shrinks below 0.4 mm, obvious bridging appears after 15 consecutive prints. For SAC0307 Solder Paste, since the silver content is reduced to 0.3% and the copper content is increased to 0.7%, the bridging rate drops to 12.6% under the same conditions. Nevertheless, its peak temperature still needs to be maintained at 235±3°C to ensure that the IMC layer thickness of solder joints meets the specification. Although tin-bismuth Solder Paste can complete reflow at 205°C, its printing viscosity is 18% lower than that of SAC solder paste, which requires the squeegee pressure to be reduced by 0.2 MPa and the speed to be synchronously slowed down to 45 mm/s; otherwise, solder paste whiskering and edge collapse will occur in the 0.25 mm fine-pitch area. After reflow, we performed X-ray scanning on BGA devices and found that the average void rate of solder joints using SAC305 solder paste was 4.7%, while the void rate of tin-bismuth solder paste soared to 19.3% under the same reflow profile settings. The root cause is that bismuth suppresses the Flux volatilization path and aggravates bubble retention. During mass production line verification, it was also observed that SAC0307 solder paste is more friendly to stencil life. After 120,000 consecutive prints, the aperture size change was only 0.8 μm, which is far better than the 2.3 μm attenuation of SAC305. However, its solder joint shear force decreased by 17.5% after 500 cycles of thermal shock at -40°C, while tin-bismuth solder paste maintained 91.2% of its initial strength due to the solid solution strengthening effect. We once tested three types of solder paste on the same PCBA. When mounting 0402 resistors, SAC305 had a 0.3% tombstoning rate, SAC0307 dropped to 0.07%, and tin-bismuth solder paste instead rose to 0.8%, which is directly related to the bismuth phase reducing surface tension and weakening self-alignment capability. In the cleaning process, SAC solder paste residues require a rosin-based cleaner combined with 60°C ultrasonic cleaning for 120 seconds to be completely removed. Since tin-bismuth solder paste has more organic acid residues, the cleaning time must be extended to 150 seconds and the water washing temperature increased to 65°C. All tests were conducted in a constant temperature and humidity workshop (25±1°C, 55±3% RH). The stencil was laser-etched 30 μm nickel alloy, the squeegee hardness was set to 85A, and the reflow oven chain speed was uniformly set to 1.1 m/min.

Customer feedback indicated that the failure rate of a certain module with an MEMS sensor reached 2.1% after reflow. On-site, we switched from the original SAC305 to tin-bismuth solder paste and simultaneously lowered the peak temperature from 238°C to 207°C, and the failure rate immediately dropped to 0.03%. However, the trade-off was a 3.4-fold increase in the AOI false positive rate, because the uneven surface gloss of tin-bismuth solder joints caused algorithm recognition deviations. In another case, after a certain automotive-grade power board switched to SAC0307, although the original poor wetting issue of SAC305 at the pins of aluminum electrolytic capacitors was resolved, the solder joint corrosion rate was found to accelerate by 2.1 times during subsequent salt spray testing. SEM energy spectrum analysis confirmed that the copper-rich phase accelerated electrochemical migration. These are not details that can be reflected in parameter tables, but rather experience accumulated from daily stencil wiping, squeegee adjustment, and reflow temperature monitoring.